Cell planning methods and apparatus, and networks configured based on same

ABSTRACT

Coverage areas for wireless networks are divided into a plurality of cells that are arranged in rows. Channel frequencies are assigned to cell sectors and associated with respective antenna axes. The antenna axes of adjacent rows are alternatingly rotated, thereby reducing co-channel interference. In an example, an available bandwidth is divided into twelve channel frequencies, so that a cell cluster includes four cells. Co-channel cells associated with half the channel frequencies are assigned to cells in the first row of cells, and the remainder are assigned to cells in the second row.

FIELD

The disclosure pertains to cell arrangement in wireless communicationnetworks.

BACKGROUND

Wireless communication systems typically provide services to asubscriber coverage area based on a division of the coverage area intoareas referred to as cells. Typically the cells are further divided intosectors, and portions of the available radio bandwidth are assigned toeach of the sectors. A group of cells that uses all available channelfrequencies is generally referred to a cluster, and the number of cellsper cluster (N) is a measure of frequency reuse. Because the number ofavailable frequencies is limited, efficient frequency reuse is animportant consideration in wireless network layout. Cell layout isgenerally selected based on providing acceptable communicationthroughout the coverage area while avoiding interference between signalsassociated with the same frequency but produced in different cells. Eachcell is typically associated with a cell site at which antennas for thecell sectors are located.

FIG. 1 illustrates a so-called wide-beam trisector division of acellular coverage area 100. The coverage area 100 is divided into aplurality of hexagonal cells and an available radio bandwidth is dividedinto 12 different channel frequencies f₁, . . . , f₁₂. Three channelfrequencies are assigned to each of the cells. As shown in FIG. 1, cells102, 104, 106, 108 use frequencies f₁–f₃, f₄–f₆, f₇–f₉, and f₁₀–f₁₂,respectively, and form a first cluster 111. Thus, for the arrangement ofFIG. 1, N=4. Additional cell clusters 112, 113, 114 are provided toextend the cellular coverage area. As shown in FIG. 1, the cells areassociated with three channel frequencies and three corresponding 120degree antennas. The antennas are configured to communicate withcorresponding sectors of the hexagonal cell areas using differentchannel frequencies. For example, antennas situated in the cell 102 arearranged to have transmission/reception directions 116, 117, 118configured to service sectors 126, 127, 128, respectively. While thisarrangement reduces co-channel interference, the coverage area 100includes so-called dead spots such as the representative dead spots 120,122, 124. These dead spots correspond to off-axis portions of antennaradiation patterns and are associated with reduced radio signal strengthin comparison with other portions of the cellular coverage area 100.

FIG. 2 illustrates a cellular coverage area 200 divided according to aso-called narrow-beam trisector configuration in which each cellincludes three 60 degree directional antennas that are assigneddifferent channel frequencies and configured to serve correspondinghexagonal coverage areas. For example, the coverage area 200 includes arepresentative cluster 201 that includes so-called “cloverleaf” cells202, 204, 206, 208. The cell 202 includes sectors 210, 212, 214 that areserviced by channel frequencies communicated along axes 216, 218, 220,respectively, typically using antennas having 60 degree beamwidths. Thecluster 201 includes N=4 cells and each of the cells 202, 204, 206, 208is assigned three channel frequencies so that this configuration isassociated with 4 by 12 frequency reuse.

The coverage area 200 as divided according to FIG. 2 does not exhibitthe dead spots associated with the wide-beam configuration of FIG. 1,but exhibits different limitations. Referring to cells 230, 232, aselected channel frequency is communicated along antenna axes 231, 233in sectors 236, 238, respectively. Thus, a signal radiated along theantenna axis 233 in the cell 232 is also received in the cell 230 alongthe antenna axis 231. Thus, reuse of the channel frequency associatedwith the antenna axis 233 in the cell 232 results in so-calledco-channel interference in the cell 230. This co-channel interference iscaused by reception of a signal intended for a recipient in the sector238 but received in the sector 236.

As shown in FIGS. 1–2, the arrangement of cells in a wirelesscommunication network is generally based on reuse of channel frequenciesto increase network capacity and improve network performance.Unfortunately, cellular arrangements such as those of FIGS. 1–2 exhibitunacceptable dead zones or unacceptable levels of co-channelinterference. Thus, increasing frequency reuse to extend networkcapacity produces degraded received signal quality, and existingnetworks often exhibit dead zones or noticeable levels of co-channelinterference. Because only limited bandwidth is available for mostwireless systems, co-channel interference is a significant limitation onsystem data rate and number of subscribers served. Thus, systems andmethods that reduce co-channel interference without introducing deadzones are needed.

SUMMARY

Methods of arranging a wireless network coverage area comprise dividingan available bandwidth into channel frequencies and allocating thechannel frequencies to at least two cell cluster types. At least one ofthe cell cluster types is provided with rotated antenna axes and thechannel frequencies are associated with antenna axis directions in thecell cluster types so that each channel frequency is associated with asingle antenna axis. Cells are arranged in the wireless coverage areabased on the at least two cell cluster types. In representativeexamples, the channel frequencies and antenna axes are assigned to theportions of the wireless network coverage area based on rows of cellclusters. In specific examples, the antenna axes assigned to a first rowof cells are rotated with respect to antenna axes in a second row ofcells. According to other representative examples, two types of cellclusters are provided and the types of cell clusters are assigned threeantenna directions. In additional examples, the first set of antennaaxes is rotated by about 60 degrees with respect to the second set ofantenna axes. In other examples, 120 degree directional antennas or 60degree directional antennas are associated with the antenna axes.

Methods of assigning channel frequencies in wireless coverage areacomprise assigning a first set of channel frequencies to respectiveantenna axes of a first cell. A second set of channel frequencies isassigned to respective antenna axes of a second cell, wherein theantenna axes of the first cell are rotated with respect to the antennaaxes of the second cell. The first cell and the second cell are includedin a first row of cells and a second row of cells, respectively.

Methods of frequency reuse comprise dividing a communication bandwidthinto channel frequencies. A first set of channel frequencies is assignedto a first row of cells, and a second set of channel frequencies isassigned to a second row of cells. The second row of cells is configuredto transmit the second set of channel frequencies at angles of about 60degrees with respect to directions in which the first set of channelfrequencies is transmitted in the first row of cells. According torepresentative examples, the communication bandwidth is divided intotwelve channel frequencies f₁–f₁₂. In specific examples, the channelfrequencies f₁, f₂, f₃ and f₄, f₅, f₆ are assigned to a first cell and asecond cell, respectively, of the first row of cells and channelfrequencies f₇, f₈, f₉ and f₁₀, f₉, f₁₂ are assigned to a first cell anda second cell, respectively, of the second row of cells so that eachchannel frequency is associated with a selected antenna axis direction.

Wireless networks comprise a first cell site configured to transmitalong three or more antenna axes and a second cell site configured totransmit along three or more antenna axes. The antenna axes of the firstcell site are rotated with respect to the antenna axes of the secondcell site. In additional examples, the first cell site and the secondcell site are configured to transmit along three axes, and the axes ofthe first cell site are rotated by about 60 degrees with respect to theaxes of the second cell site.

Wireless communication systems comprise a plurality of cells of a firstcell type situated in a coverage area, wherein the cells of the firsttype are associated with a first set of antenna axes that are assignedchannel frequencies from a first set of channel frequencies. A pluralityof cells of a second cell type are situated in the coverage area,wherein cells of the second type are associated with a second set ofantenna axes that are assigned channel frequencies from a second set ofchannel frequencies. The channel frequencies of the first set differfrom the channel frequencies of the second set and the second set ofantenna axes is rotated with respect to the first set of antenna axes.In some examples, the first set of antenna axes includes three axesangularly spaced by about 120 degrees, and the second set of antennaaxes includes three axes angularly spaced by about 120 degrees.According to other representative examples, wireless communicationsystem further comprise a plurality of cells of a third cell typesituated in the coverage area, wherein cells of the third type areassociated with the first set of antenna axes and are assigned channelfrequencies from a third set of channel frequencies and a plurality ofcells of a fourth cell type situated in the coverage area, wherein cellsof the fourth cell type are associated with the second set of antennaaxes, and are assigned channel frequencies from a fourth set of channelfrequencies, wherein the channel frequencies of the first, second,third, and fourth sets are all different. In additional representativeexamples, a cell cluster consists of one cell of each of the first,second, third, and fourth cell types.

Methods of arranging a wireless communication system comprise situatinga plurality of cells of a first cell type in a coverage area andassociating the cells of the first type with a first set of antennaaxes. A first set of channel frequencies is assigned to the cells of thefirst cell type. A plurality of cells of a second cell type is situatedin the coverage area and the cells of the second type are associatedwith a second set of antenna axes that are rotated with respect to theantenna axes of the first set of antenna axes. A second set of channelfrequencies, different from the channel frequencies of the first set, isassigned to the cells of the second cell type. In other examples, thefirst set of antenna axes and the second set of antenna axes includethree antenna axes angularly spaced by 120 degrees, and each of theantenna axes is assigned a respective frequency channel. In otherexamples, methods of arranging a wireless communication system comprisesituating a plurality of cells of a third cell type in the coveragearea, wherein cells of the third type are associated with the first setof antenna axes. A third set of channel frequencies is assigned to thethird cell type. A plurality of cells of a fourth cell type are situatedin the coverage area and are assigned a fourth set of channelfrequencies, wherein the channel frequencies of the first, second,third, and fourth cell types are all different. In other examples, cellsof the first cell type and the third cell type are alternately arrangedto form at least a first row and cells of the second cell type and thefourth cell type are alternately arranged to form at least a second row.

Methods of reducing co-channel interference in a wireless communicationnetwork comprise selecting at least one cell and rotating antenna axesof the at least one cell. In some examples, the antenna axes of the atleast one cell are rotated by about 60 degrees.

Methods of arranging a wireless network coverage area comprise dividingthe wireless coverage area into at least a first row and a second row ofcells. An available frequency bandwidth is divided into twelve channelfrequencies and first set of antenna axes is assigned to the first rowof cells. A second set of antenna axes is assigned to the second row ofcells, wherein the first set of antenna axes is rotated with respect tothe second set of antenna axes. A first set of channel frequencies isassigned to the first row of cells and a second set of channelfrequencies is assigned to the second row of cells. In representativeexamples, channel frequencies of the first and second set are different,and each channel frequency is associated with a different antenna axisdirection.

Methods of arranging cells in a wireless network service area comprisedividing the network service area into a plurality of hexagonal cellsand defining sectors in the cells. A set of antenna axes is assigned tothe plurality of cells and channel frequencies are assigned to theplurality of cells. The antenna axes are rotated, and hexagonal cellsectors are associated with the rotated antenna axes. In arepresentative example, the antenna axes are rotated by about 30degrees.

These and other features of the disclosure are set forth below withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a division of a cellular network area based on awide-beam trisector configuration.

FIG. 2 illustrates division of a cellular coverage area based on anarrow-beam trisector configuration.

FIG. 3 illustrates division of a cellular coverage area into cells basedon a rotated antenna configuration for a frequency reuse factor N=4.

FIG. 4 illustrates an alternate division of a cellular coverage areainto cells based on a rotated antenna configuration for a frequencyreuse factor N=4.

FIG. 5 is a block diagram illustrating a method for arranging cells in awireless network service area.

FIG. 6A illustrates mapping hexagonal cells into cloverleaf cells usingan antenna axis rotation.

FIGS. 6B–6C illustrate frequency assignments for cloverleaf cells.

DETAILED DESCRIPTION

With reference to FIG. 3, a wireless network coverage area 300 isdivided into a plurality of cells arranged in cell rows 301–306. Anavailable bandwidth is divided into twelve channel frequencies f₁ . . ., f₁₂ that are assigned to the cells as described below. The cells 309,319, 329, 339 are divided into sectors 310–312, 320–322, 330–332,340–342, respectively. The sectors 310–312 are assigned the channelfrequencies f₁, f₂, f₃, respectively, the sectors 320–322 are assignedthe channel frequencies f₄, f₅, f₆, respectively, the sectors 330–332are assigned the channel frequencies f₇, f₈, f₉, respectively, and thesectors 340–342 are assigned the channel frequencies f₁₀, f₁₁, f₁₂,respectively. Thus, the representative cells 309, 319, 329, 339 form arepresentative cluster 316. Because the cluster 316 includes N=4 cells,and each cell of the cluster 316 is assigned three distinct channelfrequencies so the configuration of FIG. 3 corresponds to 4 by 12frequency reuse.

Other cells shown in FIG. 3 are similarly divided into sectors andassigned the channel frequencies f₁ . . . , f₁₂ to form additionalclusters. For example, cells 326–328 are assigned the channelfrequencies f₁, f₂, f₃, cells 336–338 are assigned the channelfrequencies f₄, f₅, f₆, cells 346–348 are assigned the channelfrequencies f₇, f₈, f₉, and cells 356–358 are assigned the channelfrequencies f₁₀, f₁₁, f₁₂. Cells that are assigned the same set ofchannel frequencies are referred to as “co-channel” cells. Thus, thecells 309, 326–328 are co-channel cells, the cells 319, 336–338 areco-channel cells, the cells 329, 346–348 are co-channel cells, and thecells 339, 356–358 are co-channel cells. For convenience, co-channelscells are indicated in FIG. 3 using a common shading for each set ofco-channel frequencies.

Antennas corresponding to the selected channel frequencies f₁, f₂, f₃are situated in the cell 309 and are configured to communicate with thesectors 310–312 along antenna axes 313–315, respectively. Antennas aresimilarly configured in the sectors 320–322, 330–332, 340–342 tocommunicate along respective antenna axes 323–325, 333–335, 343–345.Other co-channel cells have similar divisions into sectors andassociated antenna axes. Antennas associated with the cells can beconfigured to have 60 degree, 90 degree, 120 degree or other beamwidths. The antenna axes 313–315 of the cell 309 are rotated by about 60degrees with respect to the antenna axes 343–345 of the cell 339. Asshown in FIG. 3, antenna axes of the cells of the rows 301, 303, 305 aresimilarly rotated by about 60 degrees with respect to the antenna axesin the cells of the rows 302, 304, 306.

The arrangement of antennas in the coverage area 300 shown in FIG. 3 isassociated with reduced co-channel interference and reduced dead zones.Cell boundary regions 350, 351 can correspond to dead zones in aconventional wide angle trisector configuration such as shown in FIG. 1,but in the configuration of FIG. 3, the cell boundary regions 350, 351are situated directly along the antenna axes 315, 325, respectively.Antenna beamwidth selection is not limited by potential dead spots.Co-channel interference is also reduced. For example, the cell 309transmits the channel frequency assigned to a sector 364 of a cell 365along the antenna axis 315 that is collinear with an antenna axis 366associated with the same channel frequency in the cell 365. However, thecell 365 is distant from the cell 309 so that signals transmitted in thecell 309 are likely to be greatly attenuated and appear only at signallevels that do not produce objectionable co-channel interference.Co-channel cells that are closer to the cell 309 than the cell 365 havenon-collinear antenna axes that tend to reduce co-channel interference.For example, with respect to the cell 309, a co-channel cell 385 that isadjacent the cluster 316 has different antenna axes so that co-channelinterference between the cells 309, 385 is reduced even though they arenot widely separated.

With reference to FIG. 4, a coverage area 400 is divided into cells suchas representative cells 409, 419, 429, 439 that have associated sets420, 422, 424, 426 of antenna axes, respectively. An available frequencybandwidth is divided into twelve channel frequencies, and threedifferent channel frequencies are assigned to each of the cells 409,419, 429, 439 to form a representative cluster 416. The coverage area isdivided into cell rows 450, 452, 454, 456 that include cells similar tothe representative cells 409, 419, 429, 439. For convenience, cellsassociated with the same set of channel frequencies are similarly shadedin FIG. 4. The cell rows 450, 452, 454, 456 are configured to haveantenna axes that are alternatingly rotated. The configuration of FIG. 4used twelve channel frequencies and has a cluster size of N=4 cells andrepresents 4 by 12 frequency reuse.

In the examples of FIGS. 3–4, coverage areas are divided based on anarrangement of hexagonal cells having distinct boundaries. In operatingwireless networks, effective boundaries between cells and effective cellareas generally depend on radio signal power obtained by a mobilestation from various antennas, and thus can depend on local propagationcharacteristics. For example, a particular transmitter is selected tocommunicate with a mobile station based on power received by the mobilestation. If the received power from a selected antenna is less than areceived antenna from a second antenna, the second antenna can beselected for communication, thereby indicating an effective cell orsegment boundary. The methods and apparatus described herein areapplicable to such effective cell areas. For convenience, cells areillustrated as hexagonal, but cells in a wireless network can have othershapes.

Referring to FIG. 5, a method 500 of configuring a coverage area in awireless network includes selecting a cluster size N in a step 502 and anumber NAX of antenna axes per cell in a step 504. In a step 506, N setsof NAX channel frequencies are selected, and, based on these N sets, Ncell templates are defined by assigning sets of channel frequencies andassociating channel frequencies with antenna axes in a step 508. Each ofthe cell templates is associated with a different set of channelfrequencies and each channel frequency is assigned to only one celltemplate. A coverage area is divided into cells in a step 510, and in astep 512, the cells are assigned channel frequencies and antenna axesbased on the cell templates. Antenna axes of at least one selected typeare rotated in a step 514.

In a representative example of the method 500, a cluster size of N=4 andNAX=3 antenna axes spaced every 120 degrees are selected. These sets areused to create cell templates that are used to define cells in acoverage area. The cells of the coverage area assigned channelfrequencies and antenna axes based on the cell templates. As originallyarranged in the coverage area, each of the antenna axes is parallel toan antenna axis of each of the other cells. The antenna axes of cellsassociated with at least one cell template are then rotated by 60degrees. In the example of FIG. 3, the antenna axes associated with thecells of a selected row are rotated, and in other examples, antenna axesof cells of a selected column, or of alternate cells, or selected cellsare rotated.

The method 500 of FIG. 5 can be implemented using a personal computer,workstation, or other computer based on a series of instructionsprovided in a computer readable medium such as a floppy disk, hard disk,or other disk, or provided via a network or otherwise provided. Forconvenience, a display can be configured to exhibit a cell arrangementin a wireless network coverage area, and antenna axis rotations can beselected using the exhibited cell arrangement. In some examples, radioparameters such as radio signal loss or co-channel signal levels can bedisplayed as well so that cells can be selected for antenna axisrotation based on network communication properties.

Cell layouts based on hexagonal cells that are divided into sectors canbe reconfigured by rotating antenna axes. With reference to FIG. 6A, anetwork layout includes hexagonal cells 601, 602, 603, 604. The cells601, 603, 604 have associated antenna axes 605, 607, 608, respectively,that are arranged to communicate with cell sectors. For example, theantenna axes 607 of the cell 603 are situated to communicate withsectors 610, 612, 614. Frequency assignments to the antenna axes are notshown in FIG. 6A. The arrangement of FIG. 6A can be modified to provideso-called “cloverleaf” cell coverage by rotating antenna axes. Referringto the cell 602, antenna axes 616 can be obtained from the antenna axes605 by a 30 degree rotation. As shown in FIG. 6A, the antenna axes 616are configured to communicate with hexagonal sectors 618, 620, 622 thatfrom a cloverleaf cell. In some examples, antenna axis rotation iscombined with a cell size adjustment due to the size difference betweensectors of hexagonal cells such as the cell 601 and hexagonal sectorssuch at the sector 622.

Frequencies can be assigned to the cloverleaf arrangement of FIG. 6A invarious ways. In a representative example shown in FIG. 6B, cloverleafcells 651, 652, 653, 654 are assigned frequency sets 1, 2, 3, 4,respectively and cloverleaf cells 661, 662, 663, 664 are assignedfrequency sets 3, 4, 1, 2, respectively. FIG. 6C illustrates anotherrepresentative example of frequency allocation.

According to representative examples, a wireless network coverage areais assigned a plurality of cell clusters. A cell cluster includes cellsthat are typically divided into cell sectors, cell segments, or othercell portions that are associated with portions of the coverage area.Each such sector or segment is assigned a unique channel frequency and aunique antenna axis direction. For convenience, antenna axes can begrouped into two or more sets that are rotated with respect to eachother. In a specific example, a cell cluster includes twelve channelfrequencies that are assigned to respective cell sectors. Six of thechannel frequencies are assigned to each set of antenna axes. Assituated in the coverage area, each channel frequency is associated witha single antenna axis direction. Antenna axes or sets thereof that areconfigured so that channel frequencies are assigned to a single antennaaxis direction are referred to as unifrequency axes. In someconfigurations, antenna axes in certain cells or certain cell clustersare not rotated with respect to other antenna axes. For example, inreconfiguring an existing wireless network to reduce co-channelinterference, antenna axes directions can be rotated in selectedportions of the network coverage area to improve network performance.

It will be appreciated that the examples described above areillustrative and can be changed in arrangement and detail. We claim allthat is encompassed by the appended claims.

1. A wireless communication system, comprising: a first plurality ofantenna sites situated in a coverage area and associated with a firstset of antenna axes that are assigned channel frequencies from a firstset of channel frequencies; a second plurality of antenna sites situatedin the coverage area and associated with a second set of antenna axesthat are assigned channel frequencies from a second set of channelfrequencies, wherein the channel frequencies of the first set differfrom the channel frequencies of the second set, and the second set ofantenna axes is rotated with respect to the first set of antenna axes; athird plurality of antenna sites situated in the coverage area andassociated with the first set of antenna axes and assigned channelfrequencies from a third set of channel frequencies; and a fourthplurality of antenna sites situated in the coverage area, and associatedwith the second set of antenna axes and assigned channel frequenciesfrom a fourth set of channel frequencies, wherein the channelfrequencies of the first, second third and fourth sets are alldifferent; wherein the antenna sites of the first plurality of antennasites and the second plurality of antenna sites are alternating arrangedin a first plurality of rows and antenna sites of the third plurality ofantenna sites and the fourth plurality of antenna sites arealternatingly arranged in a second plurality of rows, and the rows ofthe first plurality and second plurality of rows are alternatinglyarranged so as to substantially occupy the coverage area.
 2. Thewireless system of claim 1, wherein each of the sets of antenna axisdirections includes three antenna axis directions angularly spaced byabout 120 degrees.
 3. The wireless system of claim 2, wherein the firstset of antenna axis directions is rotated by about 60 degrees withrespect to the second set of antenna axis directions.
 4. The wirelesssystem of claim 3, further comprising 90 degree directional antennasassociated with the antenna axis directions.
 5. The wireless system ofclaim 3, further comprising 60 degree directional antennas associatedwith the antenna axis directions.
 6. The wireless system of claim 3,further comprising 120 degree directional antennas associated with theantenna axis directions.